DRY CONVERSION: A COST EFFECTIVE AND
ENVIRONMENTALLY SUPERIOR METHOD FOR
DISPOSAL OF UF6 ENRICHMENT TAILS
S. S. Koegler and I. J. Urza
Siemens Power Corporation
A. E. Drom
Envirocare of Utah
The dry conversion process, developed by Siemens Power Corporation for production of commercial nuclear fuel, is a clean, environmentally friendly process for converting depleted UF6 tails into uranium dioxide (UO2) for safe disposal. Envirocare of Utah conducted a series of laboratory tests on actual UO2 powder produced from UF6 by the dry conversion process to determine its acceptability for disposal at the Envirocare site. The tests and process evaluations showed that the UO2 product from the dry conversion process meets all of the criteria for disposal at Envirocare's Utah disposal facility as dictated by Envirocare's federal and state licenses. Tests also indicated that the dry conversion UO2 material is very fine-grained and has significant inherent compaction characteristics. The fine-grain nature of the waste gives it improved compaction characteristics relative to a debris waste, but it must be controlled through the addition of moisture and selected additives to prevent airborne distribution during handling. The material was also evaluated to assure that it would meet chemical and radiological verification sampling and analysis as a routine component of acceptance procedures, including, NDA (gamma) and radiochemical (alpha-spectral) analysis.
The Department of Energy and United State Enrichment Corporation have an inventory of over 500,000 tons of depleted uranium fluoride (UF6 ) "tails" left from the enrichment of uranium for United States nuclear reactors. The dry conversion process, developed by Siemens Power Corporation for production of commercial nuclear fuel, is a clean, environmentally friendly process that can be used to convert the depleted UF6 tails into uranium dioxide (UO2) for safe disposal. Laboratory tests and process evaluations show that the UO2 product from the dry conversion process meets all of the criteria for disposal at Envirocare's Utah disposal facility.
SPC DRY CONVERSION PROCESS
Siemens Power Corporation has developed and is operating a unique "dry" conversion process for converting uranium hexafluoride to uranium dioxide. The process is operating in Siemens' nuclear fuel fabrication facilities in Lingen, Germany (400 ton/year of low-enriched uranium) and Richland, Washington (1200 ton/year low-enriched uranium). The patented, environmentally friendly process (U.S. Patent No. 4,830,841 issued May 16, 1989) recovers hydrofluoric acid as a byproduct which is sold to the chemical industry. Gaseous effluents are minimal and meet all required regulations.
In the SPC dry conversion process (see Fig. 1), uranium hexafluoride, which is contained in standard cylinders, is vaporized by placing the cylinders in heated autoclaves. The gaseous uranium hexafluoride is then introduced into a conversion reactor where it reacts with superheated steam to form uranyl fluoride (UO2F2) and hydrogen fluoride (HF). The UO2F2 is defluorinated and reduced with steam, hydrogen and nitrogen in the conversion reactor. The product UO2 that is removed from the bottom of the reactor is further processed through a rotary-kiln calciner to reduce the fluoride content of the product UO2 to less than 50 parts per million (ppm).
Fig. 1. Flow diagram Siemens dry conversion process.
The conversion reactor off gas, consisting of HF, hydrogen, nitrogen and steam, is passed through filters. Condensibles (HF and steam) in the off gas from the conversion reactors and the calciner are removed in the off-gas treatment system. An on-line uranium monitor measures the uranium content of the hydrofluoric acid, which is typically below 0.1 ppm. The acid is sent to storage tanks until it is loaded into trucks for transport to a chemical facility. Final cleanup of the off gas to remove the small amount of residual HF requires a caustic scrubber or a bed of calcium carbonate. In the calcium carbonate bed a small quantity of calcium fluoride is formed when the calcium carbonate reacts with the residual HF. The calcium fluoride is recycled to the steel industry. The treated off gas is then passed through high efficiency particulate air (HEPA) filters and discharged to the atmosphere.
The dry conversion process has several attributes which make it attractive for disposal of the UF6 tails. The inherent simplicity of the dry conversion process contributes to modest capital costs and low operational manpower requirements. Because the liquid and gaseous effluents are very low, extensive waste treatment facilities are not required. In fact, the current Siemens German dry conversion fuel fabrication plant is designed for zero liquid discharge and meets all of the very restrictive German TUF requlatory requirements.
DISPOSAL AT ENVIROCARE OF UTAH
Envirocare's Clive, Utah treatment and disposal facility is located 80 miles west of Salt Lake City, Utah. The site is within a naturally arid area that is environmentally zoned specifically for hazardous industries and its distance from population centers. Several forms of depleted uranium are currently managed for disposal at Envirocare. Envirocare has disposed a large number of depleted uranium contaminated soils and debris associated with target box clean-up activities from throughout the defense sector. Since 1993, Envirocare has received and disposed over 1000 tons of de-militarized depleted uranium penetrators and associated components. More recently, Envirocare pre-accepted a DOE generated depleted UO2 waste stream from the Y-12 Plant in Oak Ridge, TN.
Fig. 2. Envirocare's Clive Facility.
Waste Acceptance Testing Demonstration
Envirocare ran a series of laboratory tests on actual UO2 powder produced from UF6 by the dry conversion process to determine its acceptability for disposal at the Envirocare site. Materials to be disposed at Envirocare of Utah must meet several acceptance requirements dictated by Envirocare's federal and state licenses. One aspect of this testing involved the generation of "fingerprint" ranges that will ultimately be used as incoming shipment screening parameters. Other tests included air/water/shock sensitivity, released cyanide and sulfide, soil pH, and paint filter liquids test. All required "fingerprint" tests were successfully completed on aliquots of the dry conversion UO2. Detailed process knowledge assessments further demonstrated that the material is acceptable for disposal at Envirocare.
Waste Acceptance Process
Shipments of the dry conversion UO2 derived from UF6 enrichment tails arriving at Envirocare of Utah, Inc. will undergo chemical and radiological verification sampling and analysis as a routine component of acceptance procedures. As required by the aforementioned procedures, NDA (gamma) and radiochemical (alpha-spectral) analysis will be conducted on shipments of the processed uranium. The results of this testing will be used to compare as received activities to those manifested, and comparative analysis will be completed to ensure the material is acceptable for land disposal at Envirocare. The samples will also be tested for incoming acceptance screening parameters ("fingerprinting") and off-site chemical analytical to verify that the processed waste is acceptable for disposal in Envirocare's low-level radioactive waste cells. Although process knowledge indicates that the material does not contain heavy-metals, periodic sampling using the TCLP (Toxicity Characteristic Leachate Procedure) will be performed to verify the material is not a mixed-hazardous waste. In addition, the State of Utah has established groundwater protection standards for fluoride, copper and zinc. Specific analytical testing will be performed to insure that these standards are met prior to disposal in the LARW Disposal Cell (Low-Level Radioactive Waste).
Bench-scale Testing and Waste Conditioning
Depleted uranium materials from the defense sector previously disposed at Envirocare have been primarily debris-type wastes. Due to the poor compaction characteristics of debris-type wastes, soil materials must be added to effect requisite compaction in the disposal cell. By contrast, the dry conversion UO2 material testing conducted at Envirocare indicates that this material is very fine-grained and has significant inherent compaction characteristics. The fine-grain nature of the waste gives it improved compaction characteristics relative to a debris waste, but it must be controlled through the addition of moisture and selected additives to prevent airborne distribution during handling.
Samples of the dry conversion UO2 material were blended with predetermined quantities of additives for the purpose of determining optimum conditioning and manageability. Envirocare permit and license requirements specify that all non-debris wastes must be at least 2% moisture by weight prior to placement in the landfill cell. Therefore, moisture and other additives must be added to the processed uranium prior to receipt and disposal.
Selected additives included polymer-based absorbents, and inorganic-based materials such as activated clays. These materials were targeted due to their ability to reduce friability of a material when applied in conjunction with moisture. Testing revealed that a comparatively small quantities (<5% by weight) of additives resulted in desirable physical characteristics for disposal. Achieving optimum manageability characteristics while minimizing the amount (and cost) of additives needed will reduce overall disposal costs during the project. Following the completion of bench-scale testing of various combinations of additives, the optimum, most cost-effective approach will be selected for use throughout the project.
Shipment Acceptance and Storage
Based upon laboratory tests and existing process knowledge, the dry conversion UO2 from UF6 enrichment tails meets all current waste acceptance criteria for management at Envirocare. After receipt at the Envirocare site, the dry conversion UO2 will be stored on a permitted storage pad while awaiting off-site laboratory results. Disposal of the material will be accomplished at regular intervals upon final acceptance of the waste, although, Envirocare's Radioactive Materials License could support storage of the entire processed waste population if necessary.
Upon completion of the acceptance component of the receipt procedures, materials shipped for disposal will be removed from shipping containers and placed directly into the disposal cell. Various forms of airborne particulate monitoring (lapel, local high volume, etc.) will be conducted in accordance with the Radiation Work Permit associated with the project. All aspects of the management of radioactive waste require the use of standard personal dosimetry.
Conversion to UO2 via SPC's dry conversion process and subsequent disposal at Envirocare's Clive, Utah waste disposal facility is an attractive option for DOE's UF6 enrichment tails. The dry conversion process is simple and requires a relatively small investment in pollution control equipment. These factors contribute to low capital investment and operating costs. Laboratory tests confirm that the dry conversion UO2 product is suitable for disposal at Envirocare's facility with only minor quantities of additives. Other aspects of the disposal, including receiving requirements, environmental monitoring, and container management, have been evaluated and support an anticipated successful and cost-effective project implementation.